Recent advances in combustion technology have employed the use of high velocity gas injection into a combustion zone to carry out combustion with reduced nitrogen oxides (NO.sub.x) generation. Nozzles with relatively small diameters are employed in order to achieve the high velocities. The high gas velocities cause furnace gases to be aspirated or entrained into the high velocity gas which has a dampening effect on NO.sub.x generation.
A problem with high velocity gas injection into a combustion zone is that material within the combustion zone, which may comprise particulate matter and condensable vapors, causes the nozzles, which have small openings to begin with, to foul or corrode as the combustion zone material contacts the nozzle. The furnace gases also tend to be quite hot, on the order of 1000.degree. F. or more, which exacerbates the fouling and corrosion problem. This problem becomes particularly severe when the furnace temperature exceeds 2200.degree. F. The fouling causes the jets issuing from the nozzles to be redirected, creating poor heat delivery to the charge and also requiting frequent maintenance which is costly and interrupts furnace production.
One way of dealing with this problem has been to provide a large amount of water cooling to the nozzle so as to prevent high temperature corrosion or melting. However, a water cooling system is complex to operate and does not address the fouling problem where the furnace atmosphere has a high particulate content. Moreover, water cooling can escalate the corrosion and fouling problems when the furnace atmosphere contains condensable vapors.
It is known that temperature effects on a nozzle may be ameliorated by recessing the nozzle in a cavity communicating with a combustion zone. However, a relatively large recess is required to achieve a significant beneficial effect. With high velocity gas injection, such a large recess may be detrimental because a large amount of corrosive furnace gas may be drawn into the cavity. Furthermore, this results in a reduction in the gas jet velocity. Thus, while the nozzle avoids temperature induced damage, this is offset by increased damage caused by contact with corrosive furnace gas drawn into the cavity.
It is known that nozzle fouling may be reduced by providing an annular flow of gas at the nozzle face. The annular gas flow serves to block furnace gases and particulate matter from contacting the nozzle, particularly when the nozzle is recessed in a cavity communicating with the main combustion zone. However, such an arrangement is very sensitive to nozzle concentricity. For example, small changes caused by nozzle movement, an imperfectly centered nozzle, uneven refractory wear or material buildup in a burner port or cavity will significantly alter the annular gas flow and may result in poor nozzle protection.
Accordingly, it is an object of this invention to provide a nozzle which may be employed in a gas injection system and which will enable effective gas injection with reduced fouling caused, for example, by the contact with combustion zone material with the nozzle.
It is another object of this invention to provide a method for injecting gas into a receiving zone such as a combustion zone while reducing the amount of nozzle fouling caused by, for example, combustion zone material.